Apex domains cannot have CNAME records.
Question and response
Question
dig voipgateway.org. NS
Answer
voipgateway.org. 5 NS ns1.backbone.ch.
voipgateway.org. 5 NS ns2.backbone.ch.
voipgateway.org. 5 NS ns5.dnsmadeeasy.com.
voipgateway.org. 5 NS ns6.dnsmadeeasy.com.
voipgateway.org. 5 NS ns7.dnsmadeeasy.com.
Authority
The response has no authority records.
Additional
ns1.backbone.ch. 5 A 212.117.192.128
ns2.backbone.ch. 5 A 212.117.221.128
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0